The present disclosure generally relates to heat treatment for age-hardenable aluminum casting alloys and, in particular, relates to heat treatment for aluminum casting alloys by direct quenching from solution treatment to the age temperature.
Aluminum silicon alloy castings are typically produced in high volume for diverse applications. In many of these applications, for example cylinder blocks and heads, transmission castings, and the like, the castings may be quite complex. To obtain adequate physical properties such as tensile strength, elongation, and hardness, aluminum silicon castings are generally subjected to a heat treatment.
The most popular Al—Si casting alloys (e.g. 319, 356, 390) are strengthened through the mechanism described as age hardening or precipitation strengthening. The process usually consists of three steps; first, the alloying elements are dissolved into the aluminum solid solution at an elevated temperature. This step is called the solution treatment and is usually performed as a separate operation from the casting process. After solidification, the casting is removed from the mold and then placed in a separate furnace to be reheated to a temperature just below the solidus and held for a period of time sufficient to dissolve precipitates and saturate the a aluminum phase with solute atoms (usually copper (Cu) and/or magnesium (Mg)). In addition, some spheroidization of the insoluble particles (such as silicon) will accompany “solutionizing.”
Following solutionizing, the casting is rapidly cooled during the second step of the precipitation strengthening process, termed “quenching.” The quench must be rapid enough to restrict diffusion and prevent the solute atoms from precipitating out of solution. A requirement of effective solute elements is that the maximum solubility in aluminum must increase with temperature, so that when the temperature is rapidly lowered, the aluminum will contain more than the equilibrium solute content and become “super-saturated.” The super-saturated state is a non-equilibrium state. Since the super-saturated aluminum composition contains more than ten times less solute atoms than the precipitate, solute atoms must cluster together to form regions of higher solute concentration and leave other areas of reduced solute concentration before a precipitate can form.
The third step is the aging step. If performed at room temperature, the aging step is called natural aging. If performed at elevated temperature, the aging step is known as artificial aging. The difference between the equilibrium solute concentration in solution at the solution temperature and the equilibrium solute concentration in solution at the aging temperature provides the driving force for the precipitation reaction. The lower the aging temperature, the higher is this difference and therefore the higher is the driving force. Conversely, the lower the temperature, the lower is the atomic mobility.
Thus, the precipitation reaction is governed by the trade-off between the compositional driving force against the temperature-controlled atomic mobility. Some precipitation occurs even at room temperature. At low temperature, the compositional driving force is high, but since the atomic mobility is low, the diffusion of solute atoms is slow and therefore the precipitation reaction is sluggish. At higher temperatures, the atomic movement is amplified making cluster-formation more rapid, but the compositional driving force is lower, resulting in a lower quantity of precipitate forming.
The choice of aging temperature in conventional heat treatment is a trade-off between reaction rate and the total amount of precipitate formed. The hardness and strength of the component is strongly controlled by the amount of precipitate formed. During aging, the casting is reheated to an intermediate temperature to nucleate the strengthening precipitates. The precipitation reaction itself is a multi-step process, causing the strength and hardness of the casting to rise with time and temperature through some peak hardness value, and then decrease again. When the aging temperature is increased, peak hardness is obtained in a shorter time, but at some expense to the level of peak hardness. Thus, there is an optimal combination of temperature and time resulting in an optimum compromise between peak strength and process time constraints.
Control of each of the above steps is vitally important to achieving the combination of strength and ductility for the particular service application. Some castings are purposely aged at higher temperatures or for longer times to obtain a condition past peak hardness. This “overaged” condition exhibits a lower tensile strength than the peak aged condition, but the increase in tensile elongation (damage tolerance) and dimensional stability can be more important than strength in many applications.
The precipitation reaction involves a diffusion-controlled agglomeration of atom clusters to form zones rich in solute. At a later stage, a discrete phase precipitates from this zone. This clustering and precipitation causes strength to increase by the increase in localized lattice strain. Still later, the precipitates grow in size until the total system energy can be decreased by formation of an interface. At this point, the particle becomes an incoherent phase and the lattice strain decreases significantly with an accompanying drop-off in hardness and tensile strength. The precipitation of the particles is also accompanied by changes to the physical dimensions of the casting with time at temperature. Therefore, for applications with critical dimensional tolerances, the casting is heat treated past the peak hardness to the point where most of the dimensional change has occurred and then it is machined to the required dimensions.
The conventional heat treatment of aluminum castings is an energy- and capital-intensive process that can involve up to two days or longer of in-process inventory at any given time.
The precipitation process is driven by the balancing of compositional driving force against the atomic mobility since each are affected by the temperature in the opposite direction. As the precipitates begin to form, the hardness and strength increase with time at temperature and the ductility decreases due to an increase in lattice strain energy created by the atomic spacing mismatch between the precipitate and the matrix.
As the precipitates grow, the local strain at the precipitate-matrix interface increases until it reaches a maximum at which the system energy can be reduced by breaking the bonds between the precipitate and the matrix, forming a phase boundary. As more precipitates become separated from the matrix by these boundaries (decoherent with the matrix), the mismatch stress are relieved thereby decreasing the hardness and strength and increasing the ductility. Thus, the common observation is that for a given microstructure, the hardness and strength vary inversely with the ductility.
Conventional heat treatments typically require a quench to approximately room temperature using separate quenching equipment before the reheating to the aging temperature. In fact, it is not uncommon for some cylinder blocks to require a 24-hour room temperature hold to reduce residual stress-induced cracking prior to reheating to the aging temperature. Reheating to aging temperature requires a tremendous amount of energy to take the aluminum alloy casting from room temperature to the aging temperature.
Therefore, there is a need for a process that eliminates wasted energy associated with reheating to the aging temperate after a room-temperature quench and that improves throughput by elimination of the room-temperature quench hold. Further, there is a need for a process that eliminates the need for separate equipment to quench to room temperature. Since the quench can be less severe and the temperature interval can be less, the residual stress and cracking tendency can also be substantially reduced.